This invention relates to a method for producing a core form suitable to be employed for injection-molding a ball joint bearing to be used in a steering system of a vehicle.
A ball joint has been known which comprises a metal housing, a resilient synthetic resin or rubber bearing received within the housing and having a spherical bearing surface and a ball stud having its spherical portion encircled by the spherical bearing surface under a pre-loaded condition. It is necessary that the bearing surface of the bearing for the ball joint of the type referred to above precisely contact the spherical portion of the ball stud under a pressure, (for example, over 75% of the contact ratio with respect to a standard surface) and the bearing surface has a mirror finish. Such a bearing has been conventionally produced by either injection molding or compression molding. However, as is well known in the art, when such a bearing is produced by injection molding, even if a core form which has been suitably designed based on the calculation of anticipated thermal shrinkage of plastic material of the bearing with respect to standard dimensions of the bearing product is employed, it is very difficult to injection-mold the bearing to its prescribed precise dimensions due to thermal deformation and partial shrinkage of the bearing material. Therefore, a core form to be employed in the injection molding of such a bearing has been conventionally produced by preliminarily making a core form from a blank, injection-molding a bearing employing such a core form, inspecting the molded bearing for any defect or irregularity of the core form and if any, correcting or eliminating the defect or irregularity based on the result of the inspection and then molding a bearing employing the corrected core form. The inspection, correction and remolding procedures are repeated until a desired bearing with the specifications can be molded.
Therefore, a core form of the above type can not be satisfactorily employed in the injection molding of the bearing even if the core form has been formed with a perfectly circular configuration and the core form has to be often modified after a series of bearings have been molded employing such a core form which has been repeatedly modified during the entire molding operation. Thus, a core form which is to be employed in the injection molding of a bearing is required to have not only a truly circular configuration, but also a precisely dimensioned spherical surface.
I previously proposed a bearing as disclosed in the copending laid-open Japanese Patent Application Publication No. 72551/1973 and such a bearing is shown in FIGS. 1 and 2 of the accompanying drawings for reference. The bearing 12 is provided in its inner or bearing surface 26 with a plurality of circular isolated oil retention recesses 25 for holding lubricant oil therein whereby the bearing can always provide a uniform and smooth rotation torque and exhibit a satisfactory impact absorbing property and also eliminate the necessity for oil replenishment. In order to produce such a bearing, it is necessary that the outer periphery of the core form to be employed in the injection molding of the bearing to provided with a plurality of reliefs adapted to form the oil retention recesses in the bearing. It is very difficult to produce such a core form by the conventional metal core form production method which requires cutting and grinding steps.
One conventional method for producing a tool steel core form will be now described referring to FIG. 2 of the accompanying drawings. According to this conventional method, first of all, a spherical core blank having the radius of curvature corresponding to the radius of curvature R.sub.2 of an imaginary circle described by the bottoms of oil retention recesses 25 to be formed in a bearing 12 is formed; then an acid resisting material is applied onto the outer periphery of the spherical portion of the core blank at locations corresponding to the locations of the oil retention recesses to be formed in the inner or bearing surface of the bearing (the so-called masking) and finally, the rest of the outer periphery of the spherical portion of the core blank other than the acid-resisting material applied or masked area is subjected to acid or electrolytic corrosion so as to produce a bearing molding core form having a spherical portion of the radius of curvature which corresponds to the radius of curvature R.sub.1 of the inner or bearing surface 26 of the bearing 12. However, a bearing formed by the use of the core form produced from such a method very often fails to have the bearing or inner surface 26 having precise dimensions because of the reasons referred to above. According to another conventional method for producing a bearing molding core form, a core form blank is first precisely processed to a true spherical shape so as to correspond to the radius of curvature R.sub.1 of the bearing surface 26; then holes are drilled in the outer periphery of the spherical portion of the core form blank in locations corresponding to those of oil retention recesses 25 to be formed in the bearing 12 which will be formed by the use of the core form and finally, steel pins having a suitable size to be snugly fitted in the holes are driven into the drilled holes to provide projections for producing the oil retention recesses 25. When a bearing is injection-molded by the use of the thus formed core form, the material for the bearing is injected about the core form to thereby form oil retention recesses 25 in the bearing or inner surface of the bearing while the bearing is being molded. However, the bearing molding core form produced by the method referred to just above usually does not have sufficient strength and also it has disadvantages that the pins can not be easily and precisely fitted in the drilled holes; further, it requires a rather long processing time resulting in high production cost.